v2 -> v3: change the value of ret only if it's positive
v1 -> v2: modify the comment of ufshcd_set_dev_pwr_mode
The return value of ufshcd_set_dev_pwr_mode is given to
device pm core. However, the function currently returns a result
in scsi command and the device pm core doesn't understand it.
It might lead to unexpected behaviors of user land. I found
the return value led to platform reset in Android.
This patch is to use an generic code for SSU failures.